This invention relates to a method of controlling an aluminum electrolytic cell, and more particularly a method of stably operating an aluminum electrolytic cell while maintaining the temperature thereof at a constant value.
As is well known in the art, aluminum is prepared by electrolytically reducing alumina in an electrolytic bath consisting mainly of cryolite. Since the alumina dissolved in the electrolytic bath is consumed as a result of an electrolytic reaction it is necessary to continuously, or at a definite interval feed alumina into the electrolytic bath. When the concentration of alumina in the electrolytic bath decreases below a certain critical limit, a so-called anode effect phenomenon appears in which the voltage of the electrolytic cell rapidly rises to 30 to 50 V. While the anode effect persists, since the normal electrolytic reaction is impaired, it is necessary to supply alumina to eliminate the anode effect. Though the anode effect can be taken as an index for supplying alumina, it accompanies a power loss, increases the job, and causes dispersion and evaporation of fluorides into surrounding atmosphere. For this reason, it has been the common practice to prevent the occurrence of the anode effect by continuously, or at a predetermined interval, supplying alumina or by supplying alumina by anticipating the occurrence of the anode effect so as to limit the frequency of occurrence of the anode effects to a permissible number.
It is advantageous to operate an electrolytic cell at a constant temperature because overheating thereof results in a decrease in the current efficiency. Moreover, as the thickness of a so-called self-lining layer formed on the wall of the electrolytic cell in contact with the electrolytic bath due to solidification thereof decreases or disappears with the result that molten electrolytic bath comes into direct contact with the wall surface of the cell thus corroding the wall surface and shortening the operating life of the electrolytic cell.
Conversely, too low temperature of the electrolytic bath increases the thickness of the self-lining to disturb supply of alumina and taking out of the formed aluminum. This also decreases the solubility of alumina so that the alumina supplied would sink and deposit on the bottom of the electrolytic cell without being dissolved. Consequently, the concentration of the alumina dissolved in the electrolytic bath decreases thereby causing frequent anode effect. As above described, too high and too low electrolytic cell temperatures result in troubles.
The temperature of the electrolytic cell during the operation is determined by an electric energy supplied thereto. On the other hand, since the current of respective electrolytic cells belonging to the same pot-line is the same and substantially constant, the temperatures of respective cells are determined by respective cell voltages. Each cell voltage varies depending upon the distance between the bottom surface of an anode electrode and the upper surface of molten aluminum in each electrolytic cell, that is the interpolar distance, and the cell voltage increases with the interpolar distance. Consequently, it is possible to maintain the bath temperature at a substantially constant value by measuring the bath temperature and by raising the anode electrode to increase the interpolar distance where the bath temperature decreases below a standard value and vice versa.
However, measurement of the bath temperature utilized in such control is relatively difficult. In an aluminum electrolytic factory, since several tens or several hundreds of electrolytic cells are operated simultaneously, it is not only expensive to install independent temperature measuring devices for all cells but also increases the cost and labor of maintenance and inspection of such large number of temperature measuring devices.
We have made investigations to find out a novel method of maintaining the temperature of an electrolytic bath at a constant value by adjusting the quantity of electric power supplied to the cell based on a readily obtainable index without directly measuring the temperature of the electrolytic cell and found that the temperature of the bath can be maintained at a constant value and the operation of the cell can be stabilized where the electric power supplied to the electrolytic cell is adjusted in accordance with the variation in the quantity of alumina presenting in the aluminum electrolytic bath or a metal in a solid state.